On the body mount, the "*" contact is
slightly recessed. On the lens mount,
"*" is a small pin whose head can be
raised or lowered depending on the
position of the lens aperture ring.
When a numeric f-stop is selected, the
pin's head is lowered so that it does
not make contact with body's "*"
contact; when the "A" aperture setting
is engaged, the pin's head is raised,
and it makes electrical contact with
the body's "*" contact.

This explains the strange behavior of the extension tube. If I use the 20mm tube the camera always thinks that the aperture ring of the lens ( SMC PENTAX-A 50mm f2 in my test ) is set to "A" setting. It also explains why my camera+12mm+20mm+lens setup was not working the same way as my camera+20mm+12mm+lens setup.

Is it some kind of bug (they mixed up the tubes)? Or maybe it's just an undocumented feature of the extension tube set to have different elements? Is there a camera+lens combination when the non-recessed pin of 20mm has advantages?

It at least looks like the reverse is true of the pin at the far right: recessed in the upper photo, but not in the lower. I'm not sure what's transmitted on that pin and/or what its being recessed does, but you may need to take both into account for it to make sense...
–
Jerry CoffinFeb 5 '11 at 16:35

Thanks for pointing out, I missed that difference. According to the K-mount page that pin is for K-F mount so hopefully it's not relevant here.
–
asalamon74Feb 5 '11 at 16:45

I'm looking at tubes to buy for pentax - does this mean that they won't set aperture at all, or that they will in one config, but not another
–
rfuscaMar 26 '14 at 1:11

Thanks for posting your response from Soligor. Given their problems with controlling their manufacturing processes and their apparent lack of concern about it, I think I'll look to another manufacturer if / when I go looking to buy a set.
–
MikeMar 23 '11 at 23:00

I've thought about this several times, and I'm going to recommend that you take the set of extension tubes back. At the very least, email the company that made them and ask if it's supposed to be that way. They do say that their products are guaranteed for 2 years. Their contact info is on the bottom of that page. I don't know where you bought it from, but if online, there should be a customer service that can help you, or just walk in the store.